I'm trying to simulate a large group singing in unison...different voice qualities, but absolutely no harmony. i want to do this from a single audio vocal track. Is TC-Helicon the best way to do this, or are there other tools to investigate?

Check out this plug in called Clone Ensemble.
Found it on another forum post here.
Just bought it yesterday, $25, will see how well it works.
Has a free trial version with limited adjustments.

The demos onsite were pretty impressive, check them out to see if it's what you are looking for.

http://www.cloneensemble.com/

Have a good one,
Carkins

TC Helicon will work, and so will other software. For me personally i use a combo of a TC Helicon hardware unit, and just singing the other tracks.